Chapter 35: Water and Sugar Transport in Plants

35.1 Water Potential and Water Movement

• Biologists use the term water potential to indicate the potential energy that water has in a particular environment compared with the potential energy of pure water at room temperature and atmospheric pressure.
• The tendency for water to move in response to differences in solute concentrations is determined by the solute potential.
• The force exerted by the wall is called wall pressure. 
• As water moves into the cell, the pressure of the fluid contents inside the cell, known as turgor pressure, increases until wall pressure is induced.
  • Cells that are firm and experience wall pressure are said to be turgid.
• Pressure potential refers to any kind of physical pressure on the water.
  • ==Inside a cell, the pressure potential consists of turgor pressure and, in the opposite direction, wall pressure.== 
• In most cases, water potential is highest in soil, medium to high in roots, low in leaves, and very low in the atmosphere. 
  • This situation sets up a water-potential gradient that causes water to move up through the plant.
  •  ==To move up a plant, water moves down the water-potential gradient that exists between the soil, its tissues, and the atmosphere==. 
  • When it does so, it replaces the water lost through transpiration.

35.2 How Does Water Move From Roots to Shoots?

• Endodermal cells are tightly packed and secrete a narrow band of wax called the Casparian strip. 
  • This layer is composed primarily of a compound called suberin, which forms a waterproof barrier where endodermal cells contact each other. 
  • ==The Casparian strip blocks the apoplastic route by preventing water from moving through the walls of endodermal cells and into the vascular tissue.==
  • The Casparian strip does not affect water and ions that move through the plastic route. 
• The movement of ions and water into the root xylem is responsible for the process known as root pressure.
• In certain low-growing plants, such as strawberries, enough water can move to force water droplets out of the leaves by a phenomenon known as guttation.
• Surface tension is a force that exists among water molecules at an air-water interface. 
• Adhesion is a molecular attraction among dissimilar molecules.
  • In this case, the water interacts with a solid substrate- such as the glass walls of a capillary tube or the cell walls of tracheids or vessel elements-through hydrogen bonding. 
  • ==Water molecules are pulled upward as they bond to each other and adhere to the side of the tube.== 
• Cohesion is molecular action among like molecules, such as the hydrogen bonding that occurs among molecules in water.
  • ==Because water molecules cohere, the upward pull by adhesion is transmitted to the rest of the water column.==
  • The water column rises against the pull of gravity.
• The leading hypothesis to explain long-distance water movement in vascular plants is the cohesion-tension theory, which states that water is pulled from roots to the tops of trees along a water-potential gradient, via forces generated by transpiration at the leaf surface.
  • ==This process relies on two of the forces involved in capillary action, namely, cohesion and tension.==

35.3 Transduction of Sugars

• Translocation refers to the movement of sugars by bulk flow in multiple directions throughout a plant-but specifically, from sources to sinks.
• In vascular plants, a source is a tissue where sugar enters the phloem; a sink is a tissue where sugar exits the phloem.
• Phloem consists largely of two cell types, sieve-tube elements, and companion cells.
• Sieve-tube elements lack nuclei and most other organelles. 
  • They are connected to one another, end to end, by perforated sieve plates.
• In the roots of the same plant, however, an entirely different mechanism is responsible for unloading sucrose.
  • Root cells in this species have a large vacuole that stores sucrose.
  • The membrane surrounding this organelle is called the tonoplast.
  • ==It contains two types of protein pumps that work together to accumulate sucrose in the vacuole, much like the phloem loading process.==

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